Generally, the television receiver art utilizes a plurality of coupling techniques for applying a detected video signal to a display device such as a cathode ray tube. For example, AC coupling, DC coupling, AC and DC coupling combinations, and back porch clamping of the video pulse signals are a few of the more common approaches for applying the detected video signal to the display device.
Unfortunately, AC coupling is responsive to the average signal level without regard to the peak black and white signal levels. Thus, the black level varies from grey to blacker than black in accordance with the black content of the video signal while the white level is similarly uncontrolled.
Also, DC coupling tends to provide fairly good peak white levels and peak black level signals for a display so long as the black, white, and sync tip levels of the transmitted signal are properly proportioned and properly transmitted at a correct level. However, it has been found that the above-mentioned proportions and levels tend to undesirably vary from station to station and program to program.
The AC-DC combination can provide some improvement over either one of the single coupling techniques but, unfortunately, includes the above-mentioned disadvantages of both to a greater or lesser degree. Moreover, the back porch clamping of the video signal would be an ideal system so long as the transmitted black and white levels were perfect. Unfortunately, the transmitted black and white levels vary from station to station as mentioned above.